Refractory iron-base alloy resisting to high temperatures

ABSTRACT

The refractory iron-base alloy according to the invention has the following composition: % BY WEIGHT Carbon from 0.05 to 0.20 Nickel do. 30 to 40 Chromium do. 20 to 30 Columbium do. 0.2 to 2 Nitrogen do. 0.04 to 0.2 Manganese do. 0.6 to 2 Silicon do. 0.8 to 2 Iron and impurities complementary percentage Said alloy offers high resistance to creep, thermal shocks, heat fatigue and intergranular oxidation, as well as good weldability. More specifically, the proportion of the various elements comply with the formula:

United States Patent Bellot et al. Sept. 3, 1974 [541 REFRACTORY IRON-BASE ALLOY 2,570,193 10 1951 Bieber et al 75 171 c RESISTING o HIGH TEMPERATURES 3,495,977 2/1970 Denhard et al...... 75/171 X 3,512,963 5/1970 Schramm 75/170 1 ntors: Jean Bellot, Pomp y; M ch l Hug 3,516,826 6/1970 Ward et 31...... 75 171 Custines, 130th Of France 3,552,950 1/1971 Rundell et a1. 75/122 [73] Assignee: Societe Des Acieries Du Mamippompey, Paris, France Przmary ExammerCharles N. Lovell I Attorney, Agent, or FirmAlbert C. Nolte, Jr.; [22] Filed: 1972 Edward B. Hunter; C. Bruce Hamburg [21] Appl. No.: 219,141

Related US. Application Data [57] ABSTRACT [63] Continuation-impart f s 7 0 OCL 6, The refractory iron-base alloy according to the inven- 1970, abandoned. tion has the following composition: [30] Foreign Application Priority Data 3 521;

July 22, 1970 France 70.27119 SI 'EZ 23;, 8 1, Jan. 29, 1971 France 71.3098 mi m from to 1(\Tlolumbium groin to (2)2 1 1 52 US. 01 /122, 75/134 F 101111251115, 11311 0.6 13 2 Silicon from 0.8 to 2 [51] Int. Cl. C22C 39/22, C22c 39/26 Iron and impurities complementary percentage [58] Field of Search 75/122, 128, 171, 134 F;

148/31, 32 Said alloy offers high resistance to creep, thermal shocks, heat fatigue and intergranular oxidation, as

well as good weldability. [56] References Cited More specifically, the proportion of the various UNITED STATES PATENTS elements comply with the formula: 2,553,330 5/1951 Post 75/122X 3OXC%+N1%+O.5 XMI'1%+16XN%=CI% 7 Claims, No Drawings REFRACTORY IRON-BASE ALLOY RESISTING T HIGH TEMPERATURES The present invention is a Continuation-in-Part of our United States patent application Ser. No. 78,608 filed Oct. 6, 1970, now abandoned.

The present invention has for its object a refractory ironbase alloy possessing simultaneously a set of highly satisfactory properties at high temperatures comprised between 600C and l,250C, owing to which the said alloy is particularly suitable for the manufacture of parts adapted to work at high temperatures, for instance outside or inside reforming and/or cracking furnaces, under particularly severe operating conditions.

The present invention also concerns all articles or elements constituted by the said refractory alloy.

The parts working outside reforming or cracking furnaces are subjected to high temperatures, considerable temperature gradients and abrupt coolings, so that the alloys known at present, the characteristics of which are not sufficiently adapted for such use, do not afford sufficiently long working life and limit the performance of the said furnaces.

The refractory alloys usually employed for such purposes are either refractory alloys in the rolled or laminated state, comprising for instance at least 0.08 of carbon, about 32 of nickel and about 21 of chromium, or moulded refractory alloys containing for instance about 0.4 of carbon, about 20 of nickel and about 25 of chromium, the carbon content of the said alloys being generally superior to about 0.20

All the above-mentioned percentages as well as the percentages given in the following description are percentages by weight.

The working conditions of the parts or the form of the assemblies composed thereof limits the use of the known alloys of the aforesaid type, in the welded state; moreover, the creep resistance of these alloys is generally insufficient. In the particular case of more than 0.20 %-carbon moulded alloys, which are very sensitive to thermal variations, there very frequently occur premature breakages of the parts made from such alloys.

The alloy according to the present invention remedies all the aforesaid drawbacks and may be used to make parts capable of constituting assemblies of quite various forms and of being used under very severe conditions such as those mentioned above in the case of the outside of reforming and/or cracking furnaces.

Thus, in a particular embodiment of the invention, the composition of said alloy is more particularly adapted to constitute metallic parts to be used on the outside of a reforming and/or cracking furnace.

In another particular embodiment of the alloy according to the invention, said alloy is intended to be used inside the reforming and/or cracking furnaces wherein there exists an oxidizing and flame atmosphere and a temperature which may reach 1,250C, on the metallic parts used inside the furnace, instead of about l,l-l,l50C on the metallic parts used outside the furnace, the temperature of the inside metallic parts being however more stable than the temperature of the outer metallic parts.

The refractory alloy according to the present invention possesses simultaneously the following properties:

high creep resistance up to l,250C,

high resistance to inter-granular oxidation even at high temperatures in oxidizing atmospheres,

a very good general resistance to oxidation up to l,250-C for certain specific compositions, the said resistance being however quite satisfactory up to l,l50C,

high resistance to thermal shocks, due mainly to sufficient ductility and holding in course of time at working temperatures, enabling this alloy to be defonned locally when the said thermal shocks occur,

good resistanceto heat stress or fatigue,

good weldability during the carrying out of any known industrial welding processes,

good aptitude to deformability in the hot state, en-

abling shaping by way of bending without by any means worsening the weldability properties or the other properties mentionedhereabove.

The forgeability of the alloy according to the invention enables to carry out hot shaping of. rough castings and to make for instance from the latter forged, laminated or rolled parts or elements. The preservation of the main mechanical properties subsequent to hot shaping aswell as the preservation of the weldability properties enable to construct satisfactory assemblies with elementsproceeding from one and the same manufacturing process.

Said alloys are characterized by the following composition:

C 0.05 0.20 by weight Ni 30 40 do. Cr 20 30 do. Nb 0.2 2 do. N 0.04 0.2 do. Mn 0.6 2 do. Si 0.6 2 do. Ta 0 0.3 do. Ti 0 l do. Mo 0 0.5 do. Al 0 0.05 do. Pb 0 0.0l do. Sn 0 0.0] do. Zn 0 0. 0l do. Cu 0 0.25 do.

Other impurities 0 0.50 do.

Fe complementary percentage According to the preferred form of embodiment of the present invention, the respective proportions of the various elements comply with a basic formula resulting in the improvement of at least some of the following properties: creep resistance, resistance to thermal shocks, resistance to corrosion at high temperatures by oxygen, carbon, sulphur or any other corrosive element likely to come into contact with the alloy, more particularly resistance to intergranular oxidation, weldability, resistance to ageing, more particularly thermal ageing, good aptitude to deformation in the hot state enabling the said alloy to be readily shaped by way of bending, forging, etc.; such properties allow the alloy to resist at best to the contact of the combustion gases, which always have an oxidizing effect owing to the excess of air required for the combustion of the fuel used as a heating means for the said furnaces.

The aforesaid basic formula is as follows:

wherein C Ni Mn N Cr Nb Ta Ti Si and Mo represent respectively the ponderal percentage of each cited element.

Where the respective proportions of the various elements comply with the aforesaid general composition and also with said formula, suitable relative proportions for the austenitic and ferritic phases are maintained. In the said formula, the elements mentioned on the lefthand side are austenizing or gamma-producing elements, whereas the elements in the second member are ferritizing or alpha-producing elements; in the case of titanium and of molybdenum, the contents of Ti, Ta and Mo may be zero as indicated above.

Preferably, one'manages to have the following formula complied with: A B i 20 with A 30 X C By complying with the above-mentioned two formulas, maximum stabilization of the austenetic and ferritic phases is obtained and intergranular corrosion through oxidation at high temperatures, on the order of from 900 to 1,250C, is thus completely avoided.

According to one feature of the present invention, it is highly preferable, more particularly for alloys used for constituting metallic parts inside reforming and/or cracking furnaces, that the silicon proportion be comprised between and 1.8 by weight; indeed, where the proportion is less than 1.0 resistance to oxidation and recarburization is insufficient above l,l50C and it does not become excellent unless the said proportion is at least on the order of 1.2 where the silicon proportion is higher than 1.8 weldability is poor and it is preferable that this proportion be at most equal to 1.5 in order that weldability and ductility be excellent; the chromium proportion should be comprised preferably between 23 and 30 and more advantageously between 24 and 28 in order to obtain the best resistance to oxidation and corrosion without prejudice to the other properties; the nickel proportion is then preferably higher than 33 to increase the maximal temperature of use.

Preferably, the proportion of titanium should be approximately twice that of the carbon proportion, whereas the niobium proportion (in the absence of tantalum) should be at least equal to six times the carbon proportion in order to maintain adequate creep resistance irrespective of the carbon proportion within the said range (0.05-0.20

Niobium may be partially replaced by tantalum, as appears from the preceding formulas corresponding to the conditions of maximum stabilization, the proportions in this case being two parts of tantalum for one part of niobium; it will be noted that tantalum is generally present, owing to the fact that the niobium of usual grade used generally to form niobium alloys contains a certain proportion of tantalum as an impurity.

Molybdenum is an unharmful impurity provided the proportion thereof does not exceed 0.50

The Applicant has also discovered that certain elements frequently encountered among the impurities in iron, in addition to phosphorous and sulphur, the sum of which should be preferably limited to 0.05 are particularly harmful to some of the desired properties of the alloy of the invention; for this reason, the proportions of casual impurities such as aluminium, lead, tin, zinc and copper are limited respectively to 0.05 %-0.01 %-0.01 0.01 and 0.25 The effect of these elements is either to reduce cold or hot ductility, or to render hot fonning more difficult or, again, to reduce creep resistance or thermal fatigue.

Ordinary microscope and electronic microscope examination of raw cast samples of the alloys of the present invention reveals an austenitic structure in which are finely dispersed carbide precipitates of the M23 C6 type in the form of small rectangular plates, the dimensions of which are for instance on the order of 800A, or in the form of fish bones, the said structure displaying microdislocation regions surrounding the particles of thesaid precipitate, appearing in the fonn of long bands. The said examinations reveal neither ferrite nor carbide of the M 7 C 3 type.

Micrographic examination reveals that intergranular oxidation is considerably reduced as compared with the known refractory alloys considered as offeringgood resistance to oxidation in the hot state.

Forgeability, which is defined as the capability of a metal tovbe shaped through plastic deformation in the hot state without producing defects, may be determined in the case of refractory steels:

by rolling or forging an ingot for the manufacture of wires by bending extruded tubes. The steels of the present invention offer the best forgeability through rolling or forging under the following conditions:

maximum temperature during the heating 1,200C

temperature at the end forging: 950C.

They offer the best aptitude to defonnation through bending under the following conditions:

heating temperature: 1,050 to l,l00C

bending at 2 950C, preferably from l,050 through l,l00C.

The refractory alloy according to the present invention may be in the state of moulded or centrifuged articles, parts or elements which are apt to be shaped in the hot state, as mentioned above, for instance by way of bending, forging, lamination or rolling.

The fact that centrifuged tubes may be bent in the hot state while at the same time holding good creep resistance, insensitivity to inter-granular corrosion, resistance to heat stress or fatigue and good weldability, renders the alloy of the present invention extremely interesting for specific uses in the above-mentioned range of temperatures (8001,250C).

The above experimental results are confirmed by the excellent behaviour of the assemblies or parts made from alloys of the present invention, under particularly severe conditions (up to 1,250C in oxidizing atmosphere for long periods of time).

According to a first embodiment, corresponding to an alloy well-adapted to be used in metallic parts outside a cracking and/or reforming furnace the composition is as follows:

Carbon from 0.10 to 0.15 by wt Nickel from 30 to 35 do. Chromium from 20 to 28 do. Niobium from 0.5 to 1.2 do. Nitrogen from 0.05 to 0.15 do. Manganese from 0.8 to 1.2 do. Silicon from 0.8 to 1.5 do. Tantalum from 0 to 0.3 do. Molybdenum from 0 to 0.5 do. Titanum from 0 to 0.20 do. Aluminum from 0 to 0.05 do. Lead from 0 to 0.01 do. Tin from 0 to 0.01 do.

-Contmued Zinc from 0 to 0.01 do. Copper from 0 to 0.25 do. Phosphorous sulphur 0.05 70 do. Iron complementary percentage In fact, for the above-mentioned use, the carbon content is preferably between 0.10 0.15 more particularly between 0.10 '0.l3 since above 0.13 the ductility may be insufficient and below the creep resistance may be insufficient (with Ni 30-33 Cr -23 According to another embodiment of the present invention, the alloy has the following composition which is particularly suitable for constituting parts inside the cracking and/or reforming furnaces:

Cr 23 to by weight Ni 30 to 40 do. Mn 0.6 to 1.50 do. Si 1.0 to 1.8 do. N 0.04 to 0.2 do. (Nb 5: Ta) 0.5 to 1.3 do. Ti 0 to 1.00 do. Mo 0 to 0.30 do. S P 0.05 do.

the contents of the other elements complying with the general disclosure of the composition of an alloy ac- 6 7( C 0.10 by weight Ni 33 do. Cr 22 do. Si 1 do. Nb 1.1 do. Mn 1 do. N 0.05 do. Ti 0.2 do. Ta 0 do. Mo 0 do.

Fe, impurities and other elements within the general range of the alloys according to the invention.

Investigation of creep resistance Creep resistance tests have been carried out at temperatures comprised between 750 and 1,150C during periods of time of from 100 to more than 10,000 hours, with a view to determining the resistance to rupture as a result of creeping of a 0.10 %-carbon nickelchromium alloy with no addition of niobium and nitrogen in a first case and with simultaneous addition of niobium and nitrogen in a second case.

From the following table I it is seen that the value of W the magnitude characterizing the creep resistance at 830C is practically doubled by adding niobium and nitrogen, by comparing the centrifuged alloy C according to the invention with the centrifuged alloy B similar to C but containing neither niobium nor nitrogen.

Table I COMPOSITION Creep resistance N Sample at 830C Ni Cr Nb N 0' R l04h 0' R [05h A laminated or rolled 33 21 1.8 hbar 1.1 hbar B centrifuged 33 22 2.3 do. 1.5 do. C centrifuged 33 22 1.1 0.06 4.5 do 3.1 do.

cording to the invention.

More preferably the following more limitative contents are used in said alloy:

C 0.10 to 0.15 by weight Cr 24 to 28 do. Ni 33 to 37 do. Mn 0 8 to 1.2 do. Si 1.2 to 1.5 do. (Nb 1% Ta) 0.6 to 1.2 do. Ti 0.1 to 0.8 do. N 0.05 to 0.15 do.

Example I The following alloy C according to the invention is now considered:

Investigation of resistance to inter-granular oxidation Table II Creep results at 850C COMPOSITION N" Sample Rupture Ultimate Stress time elonga Ni Cr Nb N hbar hours tion B centrifuged tube 33 22 5 54 2.0 B do. do. do. 4 176 1.0 B do. do. do. 3 662 0.5 C centrifuged tube 33 22 1.1 0.06 6 718 25 C do. do. do. do. do. 5 2 925 14 C" moulded element do. do. do. do. 4 4 138 25 Table III COMPOSITION Examination of broken samples N Ni Cr Nb N lntergranular oxidation Mode of rupture B 33 22 yes intergranular without deformation C 33 22 1.1 0.06 none' transgranular subsequent to deformation It is first noted that the alloys C, C and C" according to the present invention, containing niobium and nitrogen, have a low carbon-content, of the order of 0.10 and their ultimate elongation is always at least equal to 10 in contradistinction to the alloys B having the same carbon-content but containing neither columbium nor nitrogen.

More generally, whatever the duration of the artificial ageing obtained with stress (or creeping) or without stress may be, the alloy according to the invention always has an ultimate elongation superior to l as shown by numerous tests carried out by the applicant at temperatures comprised between 750 and 950C.

It should also be noted that the alloy C according to the present invention displays no intergranular oxidation in the region of rupture of the samples, in contradistinction to the alloy B compared therewith.

It should also be pointed out that the absence of intergranular oxidation prevents embrittlement of the alloy and therefore increases the heat stress or fatigue, a fact that appears clearly from the following example.

Investigation of the heat stress or fatigue Examples II and III The alloys having the following compositions are particularly adapted to be used for very prolonged periods of time as inner construction parts or elements resisting to the severe conditions prevailing outside the reforming and/or cracking furnaces, that of Example II at a temperature of 800 to l,O00C and that of Example III at a temperature of 1,000 to l,l50C.

general disclosure of the composition of an alloy according to the invention.

Example IV Another alloy according to the present invention has the following composition:

The test carried out consisted in rapidly heating and 5 cooling the edge of triangular-section samples. g 2- y sleight The values measured or the criteria examined are as 33 8: follows: Mn 1 do. Si 1.5 do. 1. number of cycles leading to the initiation of a (Nb Ta) 12 do crack I N 0.10 do. 2. crack extension rate 8.; go. 3. nature of crack 5 i P s 2: The I'CSUItS obtained are SI'lOWll Ill 1'16 following table: Fe and impurities complementary percentage COMPOSITION Initiation Propagation Nature of Number of crack N cycles Ni Cr Nb N (l) 2) (3) 32 22 50/60 3 mm after intergranular l00 cycles C 33 22 L1 0.06 450/500 0.7 mm after transgranular 2500 cycles An intergranular crack, i.e. a crack extending be- Example V tween the grains, IS an indication of brittleness of the Another alloy according to the present invention has alloy, whereas a transgranular crack is characteristic of good ductility and aptitude to mechanical consolidation; it is thus obvious that the alloy according to the invention retains good ductility and good mechanical properties subsequent to thermal ageing; otherwise stated, it therefore ofiers high resistance to heat stress or fatigue.

It is necessary to note that during the abovementioned creep or fatigue tests, the samples were subjected to such stresses as to render rupture or cracking inevitable, whereas under industrial working conditions even as severe as those mentioned above for reforming and/or cracking furnaces, the stresses have neither the same intensity nor the same frequency.

In any case, the aforementioned tests show that the behaviour of the alloys B (or A) on the one hand and the alloys C according to the present invention, on the other hand, are quite different. The said tests reveal the favourable influence of simultaneous addition of niobium, silicon and nitrogen into an iron-base nickel chromium alloy having a low carbon content.

the following composition:

Examples VI to VIII Said alloys are particularly adapted to be used for constituting inner metallic parts of cracking and/or reforming furnaces.

71 by weight Example V1 Example V11 Example Vlll C 0.08 0.18 0.06 Ni 30.0 33.5 39.0 Cr 20.5 29 28.8 Nb 0.8 1.0 0.35 N 0.16 0.05 0.04 Mn 12 2.0 1.8 Si 0.9 1.8 1.95 M 0.2 0.4 0.38 Ti 0.] 0 0.10 P S s 0.05 s 0.05 s 0.05

(*) the contents of the other elements particularly o1 impurities comply with the general disclosure of the composition of an alloy according to the invention.

C 0.05 0.20 Ni 30 40 Cr 20 30 Nb 0.2 2

N 0.04 0.2 Mn 0.6 2

Si 0.6 2 Ta 0 0.3 Ti 0 1 Mo 0 0.5 Al 0 0.05 Pb 0 001 Sn 0 0.01 Zn 0 0.01 Cu 0 0.25

and the balance essentially iron and wherein the weight proportions of the aforesaid elements comply with the following formulas:

2. Alloy according to claim 1 wherein the weight proportion of niobium added to half the weight'proportion of tantalum is at least equal to six times the proportion of carbon.

3. Alloy according to claim 1 wherein the weight proportion of titanium is susbstantially twice that of carbon.

4. Alloy according to claim 1 containing the following elements in the following weight proportions:

C 0.10 0.15 Ni 30 35 Cr 20 28 Nb 0.5 1.2 N 0.05 0.l5 Mn 08 1.2 Si 0.8 1.5 P S 5 0.05

5. Alloy according to claim 1 containing, by weight, 0.10 0.13 of carbon, 30 33 of nickel and 20 23 of chromium.

6. Alloy according to claim 1 containing the following elements in the following weight proportions:

C 0.05 0.20 Ni 30 40 Cr 23 30 (Nb+ /Ta) 0.5 1.3 N 0.04 0.2 Mn 06 1.5 Si 10 1.8 Ti 0 1.0 MO 0 0.30 P S 0.05

7. Alloy according to claim 1 containing the following elements in the following weight proportions.

C 0.10 Ni Cr (Nb /2 Ta) N Mn Si Ti Mo lllllllll Q UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,833,358

DATED September 3, 1974 INVENTOR(S) Jean Bellot and Michel Hugo It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 9, line 44, after "Cr the sy b "1-" should read line 46, after "11", the Symbol should read i Signed and Sealed this Twenty-seventh Day Of September 1977 [SEAL] Attest:

RUTH (XMASON LUTRELLE F. PARKER Attesting Officer Acting Commissioner of Patents and Trademarks 

2. Alloy according to claim 1 wherein the weight proportion of niobium added to half the weight proportion of tantalum is at least equAl to six times the proportion of carbon.
 3. Alloy according to claim 1 wherein the weight proportion of titanium is susbstantially twice that of carbon.
 4. Alloy according to claim 1 containing the following elements in the following weight proportions:
 5. Alloy according to claim 1 containing, by weight, 0.10 - 0.13 % of carbon, 30 - 33 % of nickel and 20 - 23 % of chromium.
 6. Alloy according to claim 1 containing the following elements in the following weight proportions:
 7. Alloy according to claim 1 containing the following elements in the following weight proportions. 